1. Field of the Invention
The present invention relates to a gas barrier biaxially oriented polypropylene film for packaging excellent in transparency and moisture and oxygen barrier properties and also in the gas barrier properties after elongation, a laminate film, and a production method thereof.
2. Description of the Related Arts
Biaxially oriented polypropylene films have been widely used for packaging, since they are excellent in clarity, mechanical properties, moisture barrier property, etc. Furthermore, biaxially oriented polypropylene films coated with polyvinylidene chloride for further improving gas barrier properties and moisture barrier property are widely used as a transparent packaging material excellent in gas barrier properties. However, polyvinylidene chloride generates chlorine based gases when incinerated as waste, and is feared to adversely affect the environment. To solve this problem, films with a metal oxide vapor-deposited on them have been developed as packaging films. For example, Japanese Patent Publication (Kokoku) No. 53-12953 discloses a plastic film with silicon oxide deposited.
Furthermore, Japanese Patent Laid-Open (Kokai) No. 4-20383 discloses a plastic film with a thin amorphous aluminum oxide layer formed on it. However, the aluminum oxide layer is a substantially perfectly oxidized alumina layer, and if a biaxially oriented polypropylene film is used as a substrate for it, gas barrier properties are insufficient.
On the other hand, Japanese Patent Laid-Open (Kokai) No. 62-220330 discloses an anti-static gas barrier film in which a gas barrier layer made of imperfectly oxidized alumina containing 1 to 15 wt % of aluminum is formed on a transparent plastic substrate. However, since the distribution of aluminum metal in the vapor-deposited layer is not taken into account at all, and high gas barrier properties with a biaxially oriented polypropylene film as the substrate are not obtained.
Japanese Patent Laid-Open (Kokai) No. 5-338072 discloses a transparent gas barrier film with an imperfectly oxidized alumina layer formed on a polymer resin film such as a polyethylene terephthalate film. However, since the imperfectly oxidized alumina layer is highly oxidized, the aluminum metal content is low, and no polyolefin resin layer is formed on one side of the substrate. So, high gas barrier properties cannot be obtained.
EP-A-0437946 discloses a packaging material with a coating layer made of a uniform mixture consisting of aluminum metal and its oxide on a web substrate. However, since the aluminum metal is uniformly distributed in the imperfectly oxidized alumina layer, a film with high gas barrier properties cannot be obtained.
Even if these techniques are applied to a polypropylene film used as a substrate, the vapor-deposited layer is destroyed by elongation to greatly lower the gas barrier properties when the film is further laminated or printed for processing into a packaging material because the Young's modulus of the polypropylene film is low and because the vapor-deposited layer is poor in ductility.
As optical films and production processes thereof, U.S. Pat. Nos. 4,364,995 and 4,430,366 disclose films in which the contents of a metal and a metal oxide continuously change in the film thickness direction, and also production processes thereof. The object of the technique is to continuously produce a thin metal layer and a metal oxide layer, and the film has a thin metal layer on one side in the film thickness direction and a perfect metal oxide film on the other side. So, the film is opaque and is not transparent, not being able to be used for any application requiring transparency, hence limited in application disadvantageously.
U.S. Pat. No. 4,345,005 discloses forming an ethylene-propylene copolymer layer on the surface of a biaxially oriented polypropylene film, and evaporating aluminum on it, to improve the adhesiveness of the vapor-deposited aluminum layer. However, the film is opaque and cannot be used for any application requiring transparency.